By combining the technique of immunofluorescence (to label B cells) along with autoradiography (to label (125I)-lipid A binding cells) it has been demonstrated that 20-30 percent of splenic B cells bind lipid A and that other lymphoid cell types in spleen only marginally bind lipid A. A novel early biochemical event in the activation of thymocytes by concanavalin A has also been demonstrated. Thus freshly isolated thymus cells but not splenocytes secrete a non-immunoglobulin protein in response to concanavalin A. It has been demonstrated that lipid A induces cells, uniquely present in bone marrow, to secrete proteins other than immunoglobulins.